1. Field of the Invention
The present invention relates to a radar device in which a response signal sent from a search and rescue radar trans-ponder (SART) of a wreck ship is detected when a disaster occurs in the wreck ship at sea.
2. Description of Related Art
A search and rescue radar transponder (hereinafter, called SART) has been known as a search and rescue transceiver unit which exchanges signals with a radar device of a rescue ship. The operation of the SART mounted in a wreck ship is started when a disaster occurs in the wreck ship at sea, and the SART is set to a reception wait state to receive a question signal from a radar device mounted in the rescue ship. Thereafter, when the rescue ship approaches the wreck ship, the SART receives a question signal output from the radar device of the rescue ship, and the SART sends a response signal to the rescue ship in response to the question signal. Thereafter, in the wreck ship, to encourage crew-members of the wreck ship, the SART informs the crew-members of the reception of the question signal. Also, in the rescue ship, the response signal sent from the SART is analyzed in the radar device to detect a position of the wreck ship, and the rescue ship rescues the crew-members of the wreck ship.
FIG. 6 is a block diagram showing the configuration of a conventional radar device.
In FIG. 6, 101 indicates a conventional radar device mounted in a rescue ship. 102 indicates a signal producing unit for producing a question signal. 103 indicates an antenna for radiating the question signal and receiving both a response signal sent from an SART (not shown) and an echo of the question signal as a reception signal. The antenna 103 is always rotated to receive the response signal radiated from any direction. 105 indicates a signal receiving unit for analyzing the reception signal received in the antenna 103 by amplifying and phase-detecting the reception signal and producing an image signal indicating a position of the wreck ship. 106 indicates a circulator unit for sending the question signal produced in the signal producing unit 102 to the antenna 103 and sending the reception signal received in the antenna 103 to the signal receiving unit 105.
Next, an operation of the conventional radar device 101 will be described below.
In FIG. 6, a question signal is produced in the signal producing unit 102. In the circulator unit 106, the question signal produced in the signal producing unit 102 is sent toward the antenna 103. In the antenna 103, the question signal is radiated into the air.
When the question signal is received in the SART of the wreck ship, a response signal is output from the SART, and the response signal and an echo of the question signal are received as a reception signal in the antenna 103. The reception signal including the response signal is sent toward the signal receiving unit 105 by the function of the circulator unit 106. Thereafter, the reception signal including the response signal is amplified and phase-detected in the signal receiving unit 105 to obtain an image signal. Therefore, a position of the SART can be detected according to the image signal, and an operator can visually recognize the position of the SART. That is, a distance between the radar device 101 and the SART and a direction from the radar device 101 to the SART can be visually recognized.
The detection of the position of the SART is described in detail with reference to FIG. 7(a) to FIG. 7(c).
FIG. 7(a) shows the question signal radiated from the conventional radar device 101, FIG. 7(b) shows a frequency change in the response signal sent from the SART, and FIG. 7(c) shows the image signal obtained in the signal receiving unit 105.
As shown in FIG. 7(a), the question signal is composed of pulse waves, and each pulse wave of the question signal is produced by a carrier wave set in a frequency band xcex94F1 (MHz) centering around a frequency F1 (MHz). The pulse waves of the question signal are radiated from the antenna 103. In the SART, as shown in FIG. 7(b), the outputting of the response signal is started at a reception time of each pulse wave of the question signal. The response signal is obtained by repeatedly sweeping an oscillation frequency 12 times in a sweeping frequency band from 9500 MHz to 9200 MHz. Therefore, the response signal has 12 signal waves in each response time-period, and the frequency of each signal wave uniformly changes in a saw-toothed shape. The saw-toothed-shaped signal waves of the response signal are output to the radar device 101. The transmitting frequency band xcex94F1 of the question signal is placed within the sweeping frequency band of the response signal ranging from 9500 MHz to 9200 MHz.
Thereafter, in the signal receiving unit 105 of the radar device 101, as shown in FIG. 7(b) and FIG. 7(c), components of the reception signal having frequencies of the transmitting frequency band xcex94F1 are extracted from the reception signal including the response signal, and an image signal having a plurality of pulse waves are obtained. The pulse waves of the image signal are placed at equal time-intervals because the saw-toothed-shaped signal waves of the response signal are output from the SART at equal time-intervals. The image signal is called an SART code.
Here, because the antenna 103 is rotated at prescribed cycles, the intensity of the pulse waves of the image signal is changed at the prescribed cycles. Therefore, a direction from the radar device 101 to the SART is detected according to the intensity change of the pulse waves of the image signal. Also, because the response signal is output from the SART when the question signal output from the radar device 101 is received in the SART, a time delay occurs between the outputting of the question signal from the radar device 101 and the reception of the response signal in the radar device 101. Therefore, a distance between the radar device 101 and the SART is detected according to the time delay.
However, because the transmitting frequency band xcex94F1 of the question signal is placed within the receiving frequency band of the response signal, a large volume of sea clutter and ground clutter indicated as the echo of the question signal is included as noise components in the reception signal, a signal-to-noise ratio of the reception signal deteriorates. Also, because components of the reception signal placed in almost the same frequency band as the transmitting frequency band xcex94F1 of the question signal are extracted from the reception signal to produce the image signal, a large volume of sea clutter and ground clutter indicated as the echo of the question signal is included in the image signal. That is, the question signal is returned from the surface of the sea as the sea clutter, and the question signal is returned from the bottom of the sea as the ground clutter. Therefore, a problem has arisen that a detection performance of the position of the wreck ship is suppressed in the conventional radar device.
An object of the present invention is to provide, with due consideration to the drawbacks of the conventional radar device, a radar device in which a detection performance of the position of a wreck ship is improved.
The object is achieved by the provision of a radar device comprising a signal producing unit for producing a question signal, an antenna for outputting the question signal produced in the signal producing unit and receiving both an echo of the question signal and a response signal sent from a search and rescue radar trans-ponder in response to the question signal, a filtering unit for receiving both the echo of the question signal and the response signal received in the antenna and suppressing intensity of components of both the echo of the question signal and the response signal placed in almost the same frequency band as that of the question signal to produce a filtered response signal, a circulator unit for sending the question signal produced in the signal producing unit to the antenna and sending both the echo of the question signal and the response signal received in the antenna to the filtering unit, and a signal receiving unit for receiving the filtered response signal produced in the filtering unit and extracting components of the filtered response signal placed in a receiving frequency band, which does not overlap with the frequency band of the question signal, from the filtered response signal to obtain an image signal.
In the above configuration, because the intensity of components of both the echo of the question signal and the response signal placed in almost the same frequency band as that of the question signal is suppressed and reduced, the echo of the question signal included in the filtered response signal is considerably reduced. Also, because components of the filtered response signal placed in a receiving frequency band not overlapping with the frequency band of the question signal are extracted from the filtered response signal, the image signal hardly includes the echo of the question signal.
Therefore, noise components such as sea clutter and ground clutter indicated as the echo of the question signal are considerably reduced in the image signal. Accordingly, a signal-to-noise ratio in the image signal can be improved, and a detection performance of the position of a wreck ship due to the image signal can be improved.
It is preferred that intensity of components of both the echo of the question signal and the response signal placed in a frequency band other than the receiving frequency band is suppressed in the filtering unit to produce the filtered response signal.
Therefore, even though question signals and response signals sent from other radar devices and response signals for the question signals are received as noise components in the antenna, the noise components can be considerably reduced in the image signal. Accordingly, a detection performance of the position of a wreck ship due to the image signal can be further improved.
It is also preferred that the radar device further comprises a fixed attenuator for receiving a part of both the echo of the question signal and the response signal from the circulator unit and sending the part of both the echo of the question signal and the response signal to the signal receiving unit. Intensity of components of the other part of both the echo of the question signal and the response signal placed in almost the same frequency band as that of the question signal is suppressed in the filtering unit to produce the filtered response signal, the image signal is produced from the filtered response signal in the signal receiving unit, and the part of both the echo of the question signal and the response signal sent from the fixed attenuator and the image signal are combined with each other in the signal receiving unit.
Therefore, because the part of both the echo of the question signal and the response signal indicates peripheral conditions of the radar device, an operator can visually watch a position of a wreck ship with peripheral conditions of the radar device.
It is also preferred that the radar device further comprises a fixed attenuator for receiving a part of both the echo of the question signal and the response signal from the circulator unit and sending the part of both the echo of the question signal and the response signal to the signal receiving unit. Intensity of components of the other part of both the echo of the question signal and the response signal placed in a frequency band other than the receiving frequency band is suppressed in the filtering unit to produce the filtered response signal, the image signal is produced from the filtered response signal in the signal receiving unit, and the part of both the echo of the question signal and the response signal sent from the fixed attenuator and the image signal are combined with each other in the signal receiving unit.
Therefore, because the part of both the echo of the question signal and the response signal indicates peripheral conditions of the radar device, an operator can visually watch a position of a wreck ship with peripheral conditions of the radar device. Also, even though question signals and response signals sent from other radar devices and response signals for the question signals are received as noise components in the antenna, the noise components can be considerably reduced in the image signal.
It is also preferred that the radar device further comprises a variable attenuator for receiving a part of both the echo of the question signal and the response signal set to an adjusted electric power level from the circulator unit and sending the part of both the echo of the question signal and the response signal to the signal receiving unit to adjust an electric power level of the other part of both the echo of the question signal and the response signal sent from the circulator unit to the filtering unit. Intensity of components of the other part of both the echo of the question signal and the response signal placed in almost the same frequency band as that of the question signal is suppressed in the filtering unit to produce the filtered response signal, the image signal is produced from the filtered response signal in the signal receiving unit, and the part of both the echo of the question signal and the response signal sent from the fixed attenuator and the image signal are combined with each other in the signal receiving unit.
Therefore, because an electric power level of the part of both the echo of the question signal and the response signal is adjusted, brightness of an image of peripheral conditions of the radar device displayed in the radar device can be adjusted. Accordingly, a position of a wreck ship can be properly displayed with peripheral conditions of the radar device.
It is also preferred that the radar device further comprises a variable attenuator for receiving a part of both the echo of the question signal and the response signal set to an adjusted electric power level from the circulator unit and sending the part of both the echo of the question signal and the response signal to the signal receiving unit to adjust an electric power level of the other part of both the echo of the question signal and the response signal sent from the circulator unit to the filtering unit. Intensity of components of the other part of both the echo of the question signal and the response signal placed in a frequency band other than the receiving frequency band is suppressed in the filtering unit to produce the filtered response signal, the image signal is produced from the filtered response signal in the signal receiving unit, and the part of both the echo of the question signal and the response signal sent from the fixed attenuator and the image signal are combined with each other in the signal receiving unit.
Therefore, because an electric power level of the part of both the echo of the question signal and the response signal is adjusted, brightness of an image of peripheral conditions of the radar device displayed in the radar device can be adjusted. Accordingly, a position of a wreck ship can be properly displayed with peripheral conditions of the radar device.
Also, even though question signals and response signals sent from other radar devices and response signals for the question signals are received as noise components in the antenna, the noise components can be considerably reduced in the image signal.